1. Field of the Invention
The present invention relates to a high frequency measurement apparatus and a calibration method for the high frequency measurement apparatus.
2. Description of Related Art:
A plasma processing system has heretofore been developed in which high frequency power output from a high frequency power source apparatus is supplied to a plasma processing apparatus so as to process a workpiece such as a semiconductor wafer or a liquid crystal substrate by using a method such as etching.
FIG. 11 is a block diagram showing the configuration of a generally used plasma processing system.
The impedance of a plasma processing apparatus 400 varies during plasma processing. Accordingly, the reflected wave power reflected at an input terminal of the plasma processing apparatus 400 may cause damage to a high frequency power source apparatus 100. For this reason, in a plasma processing system A100, generally, an impedance matching apparatus 200 is provided between the high frequency power source apparatus 100 and the plasma processing apparatus 400 so that the impedance matching apparatus 200 can perform a matching operation according to the impedance variation of the plasma processing apparatus 400. Also, it is necessary to monitor the impedance of the plasma processing apparatus 400 during plasma processing, as well as high frequency voltage, high frequency current and the like at the input terminal of the plasma processing apparatus 400.
Monitoring of the plasma processing apparatus 400 is performed by using various types of high frequency parameters measured by a high frequency measurement apparatus 300 provided at the input terminal of the plasma processing apparatus 400. The matching operation of the impedance matching apparatus 200 is performed by control based on various types of high frequency parameters measured by a high frequency measurement apparatus (not shown) provided inside the impedance matching apparatus 200. The following description will be given taking the high frequency measurement apparatus 300 as an example.
The high frequency measurement apparatus 300 detects a high frequency voltage (hereinafter referred to simply as a “voltage”) and a high frequency current (hereinafter referred to simply as a “current”), determines a phase difference θ between voltage and current (hereinafter referred to simply as a “phase difference”) from the detected values, and calculates high frequency parameters including a voltage effective value V, a current effective value I, an impedance Z=R+jX (corresponding to the impedance of the plasma processing apparatus 400 because the measurement point is near the input terminal of the plasma processing apparatus 400), a magnitude of a reflection coefficient Γ, a traveling wave power Pf input into the plasma processing apparatus 400, a reflected wave power Pr reflected at the input terminal of the plasma processing apparatus 400 due to impedance mismatch, and the like.
The high frequency measurement apparatus 300 includes a capacitor capacitively coupled to a rod-shaped conductor for transmitting power to the plasma processing apparatus 400 and a coil magnetically coupled to the conductor, and detects a voltage v=√2·V·sin(ωt) with the capacitor and a current i=√2·I·sin(ωt+θ) with the coil. Also, the high frequency measurement apparatus 300 determines a voltage effective value V, a current effective value I and a phase difference from the detected voltage v and current i, and calculates high frequency parameters as described above by Equations (1) to (5) given below by using the determined values. In other words, the high frequency measurement apparatus 300 is what is called an RF sensor that includes a sensor that detects voltage v and current i and an arithmetic processing circuit that calculates high frequency parameters as described above from the detected values of the sensor.
                    R        =                              V            I                    ⁢          cos          ⁢                                          ⁢          θ                                    (        1        )                                          X          =                                    V              I                        ⁢            sin            ⁢                                                  ⁢            θ                          ⁢                                  ⁢                  Z          =                      R            +                          j              ⁢                                                          ⁢              X                                                          (        2        )                                                    Γ                          =                                                            (                                                                            R                      2                                        +                                          X                      2                                        -                    1                                                                                                      (                                                  R                          +                          1                                                )                                            2                                        +                                          X                      2                                                                      )                            2                        +                                          (                                                      2                    ·                    X                                                                                                      (                                                  R                          +                          1                                                )                                            2                                        +                                          X                      2                                                                      )                            2                                                          (        3        )                                Pf        =                                            V              ·              I              ·              cos                        ⁢                                                  ⁢            θ                                1            -                          Γ              2                                                          (        4        )                                Pr        =                  Pf          ·                      Γ            2                                              (        5        )            
Generally, metrology apparatuses and measurement apparatuses have different sensor sensitivities, and thus the detected value detected by the sensor is different from the correct value. Accordingly, a configuration is used in which a calibration parameter for converting a detected value to the correct value is obtained in advance by measuring a measurement target, which is used as a reference, and in the actual measurement, a value detected by the sensor is calibrated to the correct value using the calibration parameter and then output.
For calibration of the voltage v and current i detected by the high frequency measurement apparatus 300, for example, SOLT (Short-Open-Load-Thru) calibration is used. With SOLT calibration, first, the high frequency measurement apparatus 300 is connected to a reference load whose true impedance value has been pre-specified, and the high frequency measurement apparatus 300 measures the impedance. As the reference load, a dummy load having a characteristic impedance of a measuring system (the characteristic impedance of a transmission line that transmits high frequency waves for measurement, and generally, 50Ω or 75Ω is used), and dummy loads having impedances close to an open impedance (infinity) and a short-circuit impedance (zero), respectively, are used. Next, a calibration parameter for calibrating the voltage v and current i is calculated from the impedances of the reference loads measured by the high frequency measurement apparatus 300 and the true impedance values of the reference loads and then recorded in a memory (not shown) of the high frequency measurement apparatus 300. In the actual measurement, the detected voltage v and current i is calibrated by using the calibration parameter recorded in the memory, and then various types of high frequency parameters are calculated.
The calibration parameter recorded in the memory of the high frequency measurement apparatus 300 is calculated based on the characteristic impedance and the impedances close to the open impedance (infinity) and the short-circuit impedance (zero) that are limit values, respectively, and therefore calibration can be performed in an extremely wide impedance range.
However, the calibration parameter described above is determined such that calibration can be performed in an extremely wide impedance range, and thus the accuracy of the calibration using the calibration parameter is not sufficiently high. In other words, the calibration parameter described above is a calibration parameter with which calibration can be performed in an extremely wide impedance range but with low accuracy. However, in the case where measured values obtained from the high frequency measurement apparatus 300 are actually used to monitor the plasma processing apparatus 400, highly accurate calibration is required. For example, the plasma processing apparatus 400 emits high heat during plasma processing, and thus the ambient temperature of the high frequency measurement apparatus 300 that is disposed near the plasma processing apparatus 400 increases. Because the resistive component changes with temperature changes, if the ambient temperature when calibration was performed and the ambient temperature when the actual measurement is performed differ from each other, the calibration accuracy decreases. In view of this, it is necessary to perform calibration with high accuracy.
Also, it is often the case that, in plasma processing, the phase difference θ between the detected voltage v and current i takes a value close to (1/2)π. Accordingly, even a small error in the phase difference θ will result in an increased rate of change of cos θ, exerting significant influence on the measured values for a resistive component R of impedance and a traveling wave power Pf (see Equations (1) and (4) given above). Hence, in order to suppress errors in the phase difference θ, it is necessary to perform calibration with high accuracy.
Also, there are cases where the measured values obtained from the high frequency measurement apparatus 300 are used in an E-chuck controller or the like. The E-chuck controller controls the strength of an electrostatic chuck for fixing a wafer in a chamber of the plasma processing apparatus 400, based on the measured current value and voltage value. Accordingly, it is necessary to suppress errors in the measured current value and voltage value within an extremely small range and also to minimize difference in characteristics between devices. For this reason, the detected values of the high frequency measurement apparatus 300 need to be calibrated with high accuracy.